1. Field of the Invention
The invention relates to a lock stitch machine capable of changing its stitching mode from an ordinary overcasting mode in which the flat edge of a workpiece is overcast by overcast stitches, by shifting the needle plate tongue which cooperates with the needle and the looper downward below the throat plate, to a hemming mode in which the edge of a workpiece is rolled and stitched.
2. Description of the Related Art
In a lock stitch machine of this type, the stitch tongue is disposed fixedly on the right-hand side of the throat plate so that the upper end of the stitch tongue and the upper surface of the throat plate are included in a plane. The stitch tongue cooperates with the needle and the looper for hemming.
The needle plate tongue for overcasting the flat edge of a workpiece is capable of moving between a working position where the needle plate tongue engages in the overcasting operation and an inoperative position where the needle plate tongue is disengaged from the overcasting operation. When positioned at the operative position, the needle plate tongue is set beside the stitch tongue and the throat plate and operates in cooperation with the needle, the looper and the stitch tongue to form overcast stitches. When forming ordinary overcast stitches, the needle and the looper extend the thread between the needle plate tongue and the stitch tongue to form overcast stitches in the flat edge of a workpiece.
When hemming the edge of a workpiece, the needle plate tongue is shifted from the operative position to the inoperative position. The inoperative position is near the operative position, and the needle plate tongue held at the inoperative position does not interfere with the looper and does not obstruct the stitching work of the operator.
A lock stitch machine capable of operating in either of such different stitching modes is disclosed in Japanese Patent Publication No. Sho 54-27785. In this known lock stitch machine, the inoperative position of the needle plate tongue is below the looper, and the needle plate tongue is turned across the rocking locus of the looper when the same is shifted from the operative position to the inoperative position.
In a lock stitch machine disclosed in Japanese Utility Model Publication No. Sho 63-32702, the needle plate tongue is pressed against and held on the throat plate. Therefore, the inoperative position of the needle plate tongue is near and below the throat plate and on the front side of the throat plate with respect to the feed direction. Accordingly, when the needle plate tongue is moved forward by hand in a horizontal plane, a portion of the needle plate tongue, which is disposed beside the stitch tongue, collides with the edge of the throat plate and is bent elastically downward below the throat plate. Thus, the needle plate tongue needs to be bent elastically when the same is shifted in a horizontal plane from the operative position to the inoperative position.
In the lock stitch machine disclosed in Japanese Patent Publication No. Sho 54-27785, the looper must be moved away from the path of the needle plate tongue before shifting the needle plate tongue to avoid interference between the looper and the needle plate tongue, requiring an additional operation when shifting the needle plate tongue. In the lock stitch machine disclosed in Japanese Utility Model Publication No. Sho 63-32702, it is difficult to shift the needle plate tongue because the needle plate tongue needs to be bent forcibly and it is therefore difficult to provide the lock stitch machine with an automatic mechanism for shifting the needle plate tongue.